This invention relates to coating compositions which can be applied to thermoplastic films to provide improved moisture and oxygen barrier properties and good thermal stability. More specifically, the invention relates to coating compositions which use a polymeric binder and a nano-scale particle additive to provide improved moisture and oxygen barriers for thermoplastic films.
Polymeric films are used in many commercial applications. One particularly important application is the packaging of food products. Films employed in the food packaging industry are chosen and/or designed to provide characteristics necessary for proper food containment. Such characteristics include water vapor barrier properties, oxygen and gas barrier properties, and flavor and aroma barrier properties. In order to impart these desirable characteristics in a multi-layer film, the films are often designed with one or more barrier layers. In addition to providing barrier properties, the layers must have acceptable clarity so that the products can be viewed through the film. Prior art films that used non-halogenic layers or coatings to provide barrier characteristics suffered from poor clarity and found limited use in the food packaging industry.
Polypropylene and polyethylene films are widely used in the food packaging industry because of their desirable physical properties, such as transparency and strength. Polypropylene and polyethylene films, however, do not provide barrier characteristics that are acceptable for many food packaging applications and in the past additional barrier layers have been used to provide polypropylene and polyethylene films with the required barrier properties.
Halogenic barrier materials are commonly used in the food packaging industry to impart moisture and gas barrier properties to films which do not inherently possess these characteristics. High barrier halogenic coatings or layers, such as polyvinylidene chloride (PVDC), have been used to provide moisture and gas barrier properties in thermoplastic films that include layers of biaxially oriented polypropylene (BOPP) and/or polyethylene terephthalate (PET). PVOH and PVDC, however, are difficult to recycle because they are thermally unstable and tend to degrade rapidly under typical film reprocessing temperatures (350-550xc2x0 F.). Furthermore, the halogenic barrier materials are relatively expensive and have processing problems that have limited their effectiveness. In particular, when films containing PVDC are irradiated to promote crosslinking, they discolor above certain radiation levels. Thus, there has been a demand for a new barrier coating material that gives improved recyclability and improved processing characteristics.
Aqueous coating compositions containing, in addition to a water soluble or water dispersible film-forming polymer, one or more additives intended to impart a particular property or characteristic to the substrate to which the compositions are applied are known. The most commonly used coating compositions confer clarity, good slip, and good anti-block characteristics upon the film. In addition, coatings containing halogenic polymers, such as PVDC, have been used to impart barrier properties in films but non-halogenic coatings have not been successfully used.
It has been discovered that the problems experienced in prior art films can be overcome by the present invention which provides a substantially continuous, adherent coating for at least one surface of a thermoplastic film; the coating includes a polymeric binder and nano-scale particles.
The nano-scale particles can be selected from silica, silicate, clay, organomontmorillonite, calcium carbonate, calcined aluminum silicate, hydrated aluminum silicate, calcium phosphate, alumina, barium sulfate, magnesium sulfate, and diatomacious earth. The nano-scale particles have an average particle size of from about 1 nanometer (10 Angstroms) to about 10 nanometers (100 Angstroms), preferably from about 1 nanometer to less than 10 nanometers.
The polymeric binder can be an olefinic copolymer containing a carboxylic acid group or a metal salt-functionalized olefinic copolymer. The polymeric binder can be selected from ethylene acrylic acid, ethylene methyl acrylate and ethylene ethyl acrylate copolymers.
The thermoplastic film can contain a base substrate layer that includes a polyolefin selected from homopolymers, copolymers, and terpolymers of propylene, ethylene, butylene, and blends thereof.
The coating can be applied to the surface of a single layer film or it can be applied to a coextruded layer of a multilayer film, using techniques well known to those skilled in the art, including by gravure coating, roll coating, or spraying.
The coatings of the present invention provide non-halogenic barrier materials for thermoplastic films that are environmentally acceptable because they are readily biodegradable. The coatings also provide a cost-effective alternative to the halogenic barrier materials used in prior art films without sacrificing the high performance required when packaging food products. In addition, the coatings can be applied after the film has been fabricated; therefore, the processing problems experienced with coextruded films having halogenic barrier layers are avoided.
These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.